Frequency shift converter



E. E. ELDREDGE FREQUENCY SHIFT CONVERTER Aug. 29, 195o 2 Sheets-Sheet l Filed Feb. 20, 1947 IQ. fm du w .nu w \w W A A n 1 Q mm .r mw mw W NrNlrIEllw INVENTOR. EEZ? E LDIPEDGZ JTTOJPMEYS Aug. l29, 1950 E. E. ELDREDGE FREQUENCY SHIFT CONVERTER 2 Sheets-Sheet 2 Filed Feb. 20, 1947 Patented Aug. 29, 1950 FREQUENCY SHIFT CONVERTER Earle E. Eldredge, Westbury, N. Y., assignor to Erco Radio Laboratories, Inc.,

Hempstead,

N. Y., a corporation of New York Application February zo, 1947, seri-a1 No. 729,854

'I'he present invention relates to frequency shift carrier current telegraphy and more particularly to converters for such systems which convert the two carrier frequencies used for the mark and space signals into direct current or other suitable indicia for use with telegraphic terminal apparatus.

An object of the invention is to provide a converter in which drift of the incoming frequencies may be readily detected.

A further object of the invention is to provide such a device in which the amount of Such drift may be readily measured and in which the amount of frequency shift between the mark and space signals may likewise be readily measured.

In accordance with a feature of the invention, one of the two frequencies is suppressed by means of an adjustable rejection lter, permitting the other frequency to pass through the apparatus. The suppression of a single frequency permits the use of a filter which comprises as circuit elements only resistance and capacitance. These elements are commercially available at low cost, they are compact in their physical dimensions and are extremely stable with respect to both time and temperature in their electrical characteristics.

From another aspect, because the separation of the mark and space frequencies involves only the rejection of one of these two frequencies, thus permitting the use of a lter containing only resistive and capacitative circuits elements,the resistive elements may be made adjustable with an eX- cellent degree of precision in setting. Moreover, if desired, suitable indicating means may be provided in conjunction with the adjustable resistors and calibrated in terms of the frequency rejected. This calibration will remain stable and the frequency measuring device thus obtained will be economical in cost and light and compact in comparison to a device employing suitable calibrated standard inductors, standard capacitors or similar precision elements.

Another feature of the invention permits monitoring of the incoming signals and manual adjustment of the rejection filter to obtain the maximum degree of rejection. Having determined this setting, the frequency rejected may be measured by reference to the calibrated dial of the rejection filter. By rejecting first one frequency and then the other and noting the corresponding settings of the calibrated dial, the amount of frequency shift may be readily measured, and corrections may then be made at the transmitting apparatus, if required.

Other and further objects will become appar- 4 claims. (C1. 17a-8s) `band range of filter I3.

ent upon reading the following specification together with the accompanying drawings forming a part hereof, in Which Fig. l is a schematic circuit diagram of an embodiment of the invention.

Fig. 2 shows the frequency response characteristics of certain lters of Fig. 1.

Fig. 3 is a fragmentary schematic circuit diagram showing a modied form of the invention.

Fig. 4 is a diagrammatic perspective View of the variable resistance elements of an adjustable lter.

Referring to Fig. 1, input terminals II and I2 are adapted for connection to the output of a radio receiver or to some other source of signals in which an output is always present. This output shifts in frequency between two relatively fixed frequencies in accordance with the telegraphic mark and space signals.

Terminals I I and I2 are connected through a band-pass filter I3 of suitable impedance characteristics to the input of a limiter I4. The response characteristics of filter I3 are shown in Fig. 2, being represented therein by curve I5. The width of the pass-band is sufficient to embrace the maximum amount of frequency shift with which the apparatus is intended to be utilized. i The sharpness of cut-off at the extremes of the pass-band is preferably as great as may be obtained within the limits of good commercial practice. Sufficient damping should be included in lter I3, however, to permit the lter to respond to the shifting frequency at the keying speeds utilized without producing distortion by any tendency to sustain oscillations.

` Limiter I4, to which the signals from lter I3 are applied may be of any conventional type, and

input because either the mark signal or the space signal will always be present, the only difference between the two being in their respective frequencies rather than in their` amplitudes. The limiter, of course must give substantially uniform operation at frequencies within the pass- Limiters having these characteristics are well known, such as the grid current type for example, and the limiter therefore is not shown in detail.

In vpassing through the limiter I4, the signals duce the anode voltage to a suitable value.

Vinput of a frequency rejection lter v2li.

'may be modified in wave shape by having the peaks of the waves clipped. This produces an output resembling the output of a square wave generator and which is relatively high in harmonic content. Another band pass filter similar to filter i3 may be used or a low-pass filter may be used to remove these harmonics, but it has been found to be Sufficient in practice to use a simple tuned circuit such as the parallel inductance and capacitance combination I3. This combination should have its resonant point near the center frequency of the pass-band of filter i3 and should be fairly broadly tuned to lavoid undue sharpness of response at any frequencywithin this band.

After removal of the harmonics, the signals are applied to the grid of triode li which is connected as a cathode follower. In order that output of the apparatus may have a relatively low potential with respect to ground, the anode supply has its positive terminal grounded and the negative terminal has been designated as B-. The anode is connected to ground through a suitable dropping resistor 'I3 which is shunted by a by-pass capacitor i9. VResistor I8 serves to re- The output of cathode follower 'l'i is applied lto the The cathode follower it serves to reduce the impedance at the output of this stage to a value suitable for use with the input impedance of rejection filter 2li. Although rejection filter 2o may take any convenient form, it has been shown as a double T impedance network comprising in one of the Ts the series resistors 2l and l22 with shunt Vcapacitor 23 and in the other T, series capacitors 24 and 25 with shunt resistor 26. The

Acircuit constants are .preferably somewhat asymmetrical and so proportioned as to obtain a frevquency response characteristic having a sharp Anotch substantially as shown in Fig. 2 by curve which indicate their adjustability. With such lan arrangement, the correct .rela-tive resistance values may be maintained over a range of settings,

Yand the commonoperating shaft 'l2 for the three variable resistors may be provided with a -calibrated dial 'it and pointer 'F'l which -in turn vmay ybe calibrated to read directly the value of the frequency rejected. With an arrangement of this rtype where only a single frequency is rejected with a relatively high degree of selectivity as indicated by the notch-shaped response characteristic 27E of Fig. 2, it is important that-harmonics generated in the limiter I4 be removed by the tuned circuit -iE to an extent sufficient for .proper Yoperation of the apparatus. Filters of this character require a substantially sinusoidal wave -shape for effective rejection.

vFrom the output of rejection filter 20, the non- `rejected frequency, which is preferably the mark frequency, .passes through triode 28 which is connected `as a conventional resistance-coupled amplifier, and which serves to offset the attenu- -at'ion produced by Afilter 2E).

From the output of triode 23, the mark frequency passes lthrough coupling capacitor 29 and `.along conductor 30 to the control grid of tetrode 4 32 the output of which is connected as a transformer Coupled amplifier. Grid bias is provided by cathode resistor 33 and its associated by-pass capacitor 34. Anode potential is derived through dropping resistor 35 and its associated by-pass capacitor 36. The screen potential is obtained by the further action of dropping resistor 3`| and its associated by-pass capacitor 3B. Anode potential is applied through the primary winding of output transformer 39. A small capacitor 4G is connected across the primary winding of transformer 39 in order to reduce any possible tendency toward oscillation, particularly at higher frequencies The amplified mark frequency is rectified by the action -of twin diode 4| which is shown connected as a full-wave rectifier. Resistor 42 serves as a load resistor for this diode.

A lmonitoring jack 43 is connected across the secondary winding of transformer 39 through coupling capacitors 44 and 45. This monitoring jack is for use with a headset or other suitable apparatus as described below.

The rectified mark signals derived from diode load resistor 42 are passed through a low-.pass filter 4S. This is shown as a resistance-capacitance type of filter which is ordinarily the most economical to construct and for this reason is considered to be preferable although any other desired form of construction may be used. rThe time constants of the filter are selected to give cut-ofi` action at frequencies appreciably above the components of the maximum keying speed. The cut-ofi frequency is selected, however., to be sufficiently high so that .no appreciable distortion is introduced vin the rectified signals.

The amplifier 4i `is more fully described in the copending application of Thomas E. Goodwin, Serial No. 748,608, filed May `16, `1947, Anow vPatent N0. 2,516,201, .patented AJuly ,25, 1950. The trigger amplifier in itself forms a part lof the present invention -only in combination with the other features herein described.

The signals passed by filter 46 are applied to lthe input `of a -direct current trigger type amplifier 4l. The amplifier shown :has -been found Vto be suitable, but -any other convenient `formcf amplifier maybe used if desired.

The input is applied directly to the y.control grid of triode 48. The .biasing potential applied to the cathode -of triode -48 Iby current flowing through the series arrangement comprising vresistors 12, 71| r`and 50 is sufficient to maintain the plate current substantially .at zero. Because 'of the cut-off *condition of triode 48, the voltage drop "acrossresistor f'l is negligible and, accordingly, the-controlgrid'of tetrode 52 is maintained at substantially the-same potential as Vits cathode. This low v.grid bias -on tetrode 52 causes it =to draw a relatively `heavy plate current which fin turn fiows through resistor :50 Yand helps to maintain the cut-off condition of triode t8. Triode 48 :is `preferably of 4the high amplification low plate current type.

Upon receipt of :a Emark signal, which passes through rejection filter A2l] jand reaches 'diode 4I, a positive potential is Iproduced which is applied to the grid of triode 48. This potential is :sufficient in magnitude to cause triode 48 to 'draw an .appreciable current which flows through resistor 1U. The drop across resistor 10 places a corresponding negative bias on Ithe control grid of tetrode A52 and .reduces its normal plate lcurrent. This reduction in turn reduces the .cur- .rent fiow vthrough resistorVV-oto la somewhat t greater extent than the increase in current flow through `this resistor occasioned by the current now being drawnby triode 43. The net decrease in current flow through resistor B has reduced the grid bias of triode 4S, thus aiding the applied positive potential derived from the mark signal. The circuit constants must be so proportioned, however, that grid of triode 48 maintains control thus assuring a positive returnto the cut-oir` condition in triode 4% as soon as the positive potential derived from the mark signal is reduced. As soon as this reduction commences, the plate current of triode Q8 decreases with a resulting decrease in the bias of the control grid of tetrode 52 and a corresponding increase in voltage drop across resistor 5l). Preferably, the resistors and tube characteristics are so selected that a sharp trigger action is obtained between triode e8 and tetrode 52, the circuit switching substantially instantaneously from a cut-off condition in one tube and a conductive condition in the other to a conductive condition in the rst tube and a cut-off condition in the latter. By suitable design, a sharp clean cut response may be obtained with relatively small changes in input voltage applied to the grid of triode d8.

`The lower half of thel amplifiercr trigger circuit comprising triode da, tetrode 53 and resistors 5l, 13, 'i4 and 75 is substantially identical with theupper half previously described. Ilhe input however is derived from a tap on resis tor 5i) which is so selected that with triode 132i cut-off during the absence of a mark signal, a sufiicient positive potential will be applied to the grid of triode :i9 to maintain this tube in a conducting condition and to maintain tetrode 53 in a substantially cut-off condition. Upon response to a mark signal, the drop across resistor 5o decreases to an extent sufficient to produce `cut-ofi" of triode d and an accompanying conductive condition in tetrcde 53. Duringthe space signal, these conditions are reversed.

During the space signal an output appears across resistor 54 as a result of current iiow through tetrode 52. Upon receipt of a mark signal, tetrode 52 cuts off and current flows through tetrode 53 producing an output across resistor 55.

A polarized relay 5d, may be of any conventional construction suitable for use at the telegraphic keying speeds to be utilized. This relay is provided with two windings 6l and te which are energized by the mark and space signals respectively, being joined together and to ground and being connected in the opposite sense of polarity to the respective outputs of pentodes 53 and 52 of the direct current trigger amplifier di.

Preferably, the polarized relay 5S is removably mounted in the apparatus. This permits an increased output to be derived from terminals 56 and 5?, if desired, by removal of the relay. Any other suitable means may be utilized to disconnect the windings Si) and Ci! of relay 59 from the circuit, when external signaling apparatus is directly actuated by amplier fil. The contacts of relay 59 arg` Wired to terminals 52, 53 and dit where they may be utilized to control telegraphic terminal apparatus, as desired. If the terminal apparatus includes a source of current to be controlled by contacts then terminals e2, t3 and Eil are used. If, on the other hand, it includes a polarized control magnet, then terminals 56 and 51 are to be used, and relay 59 removed from the circuit. If the terminal apparatus is non-polarized and is controlled by a magnet without a source of energy, then either terminal 5l and ground terminal 65 are used for responsetofthe mark signal, or terminal 55 and ground terminal 55 are used for response to the space signal.

In operation, a monitoring device such` as a pair of head phones or a visual indicating device such as a cathode ray oscilloscope may be plugged in to monitoring jack @3. rIhe rejection filter 2t may then be tuned first to the mark frequency and then to the space frequency and the amount of shift read from` the calibrated dial 'iii on the filter. For use, it is then set to give the maximum elimination of the space frequency, and because of the sharply notched response curve, any drift in the incoming space frequency can be readily detected. `In addition, the amount of the drift can be measured by retuning the rejection filter and noting the new frequency from the calibrated dial l. i

A single channel has been shown from the rejection filter 2u to the trigger-amplifier 47, the space signal being `rendered ineffective in this channel and being artificially re-created by using the varying voltage drop across resistor 50. This is the simplest embodiment of my invention, and the simplicity is made possible by the uselof the rejection lter 26. It is possible however, to obtain some improvement, amounting perhaps to about one or two decibels, by using two separate channels from triode il to the output tetrodes. Such an arrangement is shown in Fig. 3. The output of cathode follower l! feeds two rejection filters Zi and 2G', one for the mark frequency and the other for the space frequency. These outputs are individually amplified by separate triodes 28 and 28 andtwo separate circuits are provided, one containing instrumentalities 32, 39, d3, 4| and l5 and the other `containing corresponding instrumentaiiues s2', es', is', ai' and as' together with their associated circuit elements. At trigger amplifier lil, the grid lead of triode de, instead of being connected to a tap on resistor 5l), is connected to the output of its individual low pass filter 45 similar to filter d6 in the same manner as triode d8. The outputs of tetrodes 52 and 53 remain unchanged, however.

Using this modified arrangement, the two rejection filters may be adjusted individually and both mark and space frequencies may be noted by observation of the two calibrated settings at all times. This modification may be readily made by a person skilled in the art and has therefore not been shown in the drawings.

I have shown what I believe to be the best embodiments of my invention. I do not wish to be limited by the specific embodiment shown but by the invention as set forth in the appended claims.

I claim:

1. In a converter for use in telegraph terminal apparatus for separating the mark and space signals employed in frequency shift carrier current telegraphy the combination of: a band pass filter adapted to pass both the mark and space signal frequencies, a limiter operatively associated with the output of said band pass filter for maintaining the intensity of said signal frequencies at a substantially constant level, an impedance network connected to said limiter to reject one of said signal frequencies, rectifying means connected to said impedance network, said last means being responsive predominantly to the signal frequency least affected by the operation of said impedance network and direct current amplifying means connected to said rectifying means to produce an output suitable for telegraphic utilization apparatus,

i2. .in a converter iorzuse .in telegraph "terminal apparatus for separating theinark-andspace signals employed :in 4frequency shift 4carrier .current ,telegraphy .the combinationoi 4a bandpass filter :adapted to. pass both said frequencies, ,a limiter Aoperatively associa-ted with the youtput `Vof said band pass lter for maintaining the intensity .of saidsignal frequencies :at .asubstantially constant level, `an impedance netwerk responsive :to the output of said limiter and Aanimated toi-eject .one

.of said signal frequencies, .rectiiying means -responsive predominantly to the .signal frequency .aflleeted to `a lesserextent by the operation of .sa-id impedance network, a low pass viiiter responsive to .said :rectifying means for 4eliminating 4requencies .above the component frequencies of the signai frequency .required vto be passed Joy said 4filter to prevent distortion of the rectified signals, iiireet current amplifying apparatus responsive to the output. of said 10W ypass ,filter and connected to produce an output vsuitable Vfor 4telegraphic utilization apparatus.

In a converter for use in telegraph terminal apparatus for separating the mark :and space signals employed in frequency vshift carrier .current .telegraphy the combination of a band pass filter adapted to pass .both said frequencies, va limiter operatively :associated with the .output of said band pass fil-ter for maintaining the intensity of .Said signal frequencies at a substantially constant level., a pair of impedance networks each adapted to ,reject one of said signal frequencies, individna-l .rectify-ing means associated with eac-h of .said impedance networks and responsive predominantly to the frequency least affected by the `operation of its associated one of said impedance networks, direct `current amplifying means each responsive to one of said -rectiiying means, and circuit means. connected to combine `the outputs of ysaid amplifying means and -to produce an output suitable for telegraphic utilization. apparatus.

,4. In la converter .for aise .in telegraph terminal .apparatus for :separating `the mark Vandrspacesgnais employed .in frequency shift carrier current telegrapny itne combination of: impedance ,net- Work .means for rejecting either Athe Yina-rk. or the ,space .signal frequency, rectifying means respon- .si-yepredominantlyito the frequency least affected by .the operation .of said impedance network means, .amplifying :means associated with :said -rectrying means `and connected to produce an output in .the .absence of leither signal frequency and .further vamplifying means .operatively .associated with the first-named amplifying means and .connected to produce an output 'in response toa condition which produces a reduction inthe output of the rst-named amplifying means.

EARLE E. ELDREDGE.

REFERENCES CITED The following references are of record in the le of this patent:

vUNITED STATES PATENTS Number Name Date 1,571,005 Hartley Jan, 26, 1926 1,791,578 Singer Feb. 10, 1981 2,164,745 Ken'tner July 4, 1939 2,211,750 Humby Aug. 20, 1940 2,312,145 Bradley Feb. 23, 1943 2,373,495 Olsen Apr. 10, 1945 2,384,456 Davey Aug. 11', 1945 A2,446,077 Crosby July 27, 1948 2,463,402 Maki Mar. 1, 1949 FOREIGN PATENTS Number Country Date 526,680 Great Britain Sept. 24, 194i) OTHER REFERENCES Reprint Proc. IRE, Sept. 1946, vol. 34, No. 9. 

